1. Field of the Invention
The present invention generally relates to mass flow controllers used in semiconductor processing systems.
2. Description of the Related Art
Conventional semiconductor manufacturing techniques may include advanced chemical and/or thermal reactions that are extremely sensitive to the processing conditions within the processing chamber. In a dry etching-type process, for example, the flow of etch gases supplied to a dry etch chamber having a semiconductor wafer positioned therein must be carefully controlled if the desired etch characteristics are to be obtained. Further, in nucleation processes that may be implemented prior to depositing tungsten monolayers in a chemical vapor deposition process (CVD), for example, the deposition chemical reaction generally begins immediately upon the reactant gases being supplied to the processing chamber. Therefore, if the flow of the reactant gases into the processing chamber is not initiated at or very near a calculated optimal flow rate for the particular chemical reaction, then the chemical reaction often yields undesirable and/or unpredictable results that may substantially reduce the device yield from the process.
In an attempt to precisely control gas flow into processing chambers and/or processing environments, conventional semiconductor processing systems have generally implemented one or more mass flow controllers (MFCs) to regulate and/or control the flow of reactant gasses into the processing environment. These MFCs generally operate by receiving a gas supply at an MFC input and outputting a regulated gas supply at an MFC output. The MFC output is generally in communication with the processing chamber/environment, and therefore, are generally used to supply a reactant processing gas thereto.
In operation, conventional MFCs generally regulate and/or control the pressure and/or volume of the gas supply at the MFC output in accordance with at least one input received from a user. FIG. 1 illustrates a conventional MFC 10 that may be implemented in a semiconductor processing system in order to control reactant gas flow into the processing system. The conventional MFC 10 may receive a reactant gas, which may be a single gas or a combination of gases, at a primary side input 11 to MFC 10. The reactant gas flowing into the MFC 10 from the primary side is generally divided into two portions, wherein a first portion flows through a restriction device 12 and the second portion flows through a flow sensor bypass tube 13. Within the flow sensor bypass tube 13 the mass flow of the gas passing therethrough is cooperatively determined by temperature sensors 14, heater 23, and a bridge circuit device 15 in communication with the temperature sensors 14. Heater 23, which may be positioned equidistant from each of temperature sensors 14, heats a constant percentage of the MFC gas flow. With no gas flow, the heat reaching each of sensors 14 is equal. With increasing flow, the gas flow stream carries heat away from the upstream temperature sensor 14 and towards the downstream temperature sensor 14. This temperature difference may be measured and is representative of the gas flow in the bypass tube 13. Therefore, since the flow of gas through bypass tube 13 is proportional to the total flow of gas through MFC 10, then the total flow of gas through MFC 10 may be determined from the mas flow of gas traveling through bypass tube 13. The determined temperature change may be converted into representative electrical signals through a bridge circuit device 15, and thereafter, the representative electrical signals communicated to an amplifier circuit 16. Amplifier circuit 16 operates to amplify the electrical signals and then communicates the representative electrical signals to a user display device 17, which may convert the signals into a format that may be viewed and analyzed by the user. Additionally, the amplifier circuit may communicate the amplified electrical signals to a control circuit 19 within MFC 10.
Control circuit 19 generally operates to control the position of the primary MFC valve 21, which essentially operates to allow gas to flow through MFC 10, via a valve driver 20 in communication with the control circuit 19. Control circuit 19 also receives an input from a user input device 18 that may operate to indicate to control circuit 19 the user""s desired flow rate. Thus, control circuit 19 may compare a measured flow rate, which is indicated by the representative electrical signals received from amplifier circuit 16, with a desired flow rate received from user input device 18. Thereafter, the control circuit 19 may adjust the position of valve 21 to increase or decrease the gas flow through MFC 10 such that wherein the increase or decrease is calculated to adjust the gas flow through the MFC closer to the desired gas flow. This process is generally termed a ranging in-type process, as the MFC valve position is adjusted towards the desired position in a dampened oscillatory manner so that the oscillation of the valve position is calculated to decrease to the desired position within a predetermined amount of time. Therefore, if the gas flow is to be increased, for example, then the control circuit would communicate to valve driver 20 to actuate valve 21 in the direction shown by arrow xe2x80x9cAxe2x80x9d. This increases the spacing between the terminating end of valve 21 and the wall of MFC 10 so that additional gas may be allowed to flow through MFC 10 in the direction indicated by arrow xe2x80x9cBxe2x80x9d. The gas passing through MFC 10 is outputted through an MFC output 22, which is generally in communication with a processing chamber or processing environment.
Although conventional MFCs are generally effective in maintaining a relatively constant gas flow once the flow is initiated, the implementation of a control circuit receiving an input from an amplifier circuit and adjusting the valve position in order to obtain a desired gas flow is generally ineffective in generating an accurate and/or predictable gas flow at startup conditions. In particular, the combination of a sensing device transmitting a signal to a valve control device results in a xe2x80x9cranging inxe2x80x9d type of operation in order to obtain the desired flow rate. Ranging-in operations, as are known in the art, generally include a process of measuring a current flow and adjusting the current flow in the direction of a desired flow. If the difference between the current flow and desired flow is substantial, then the adjustment, which is generally calculated, may also be substantial. Ranging-type operations are effective when the actual gas flow is proximate the desired flow, as the control circuit generally only has to make a minor valve adjustment to obtain the desired flow. However, in situations where the actual gas flow is not proximate the desired gas flow, then the control circuit generally attempts to substantially alter the valve position in order to bring the current flow rate to a level that is proximate the desired level. This substantial alteration in turn causes a return reaction in the control circuit, which initiates a dampened oscillatory condition that eventually results in the MFC ranging the valve position into a position that yields the desired flow rate. This condition is generally caused by a lack of gas pressure on a flow control valve during a flow startup process. The lack of gas pressure at flow startup generally operates to cause the MFC flow controller to open the flow control valve farther in an attempt to initiate gas flow at the desired rate. However, with no gas resident at the flow control valve upon startup of flow, the controller attempts to increase flow by further operating the flow control valve. Therefore, once gas arrives, the valve is too far open and the controller must compensate in the opposite direction. This effect results in the ranging in and/or oscillation conditions.
FIG. 2 illustrates an exemplary graph of the voltage applied to an MFC valve driver upon startup of a gas flow process. At approximately time equals 1 second in FIG. 2, the MFC may receive a signal to initiate gas flow at a predetermined rate of, for example, 20 standard cubic centimeters per minute (sccm). Upon receiving the signal to initiate gas flow, the MFC generally compares the current gas flow rate with the desired gas flow rate indicated by the signal to initiate gas flow. In this situation, the disparity between the current gas flow rate, which is zero as the MFC valve is completely closed, and the desired gas flow rate is at a maximum value. Upon determining this disparity, the MFC control circuit sends a voltage signal to the valve driver calculated to minimize the disparity in a very short period of time, i.e., the controller calculates a valve that will reduce the voltage/flow disparity in a minimal time period. This voltage signal is evidenced by peak 201 in FIG. 2, which is approximately 0.65 volts. This voltage, which is substantial when compared to the end result voltage of 0.3 volts for the desired flow, is calculated to rapidly open the valve so that the disparity between the desired and actual flow rates will be quickly diminished.
At a short period of time after the initial voltage signal is applied to the valve driver, the control circuit again compares the actual flow rate with the desired flow rate and determines that the voltage signal applied to the valve driver is disproportionately higher than that which is required to generate a flow of 20 sccm, as the measured flow is now greater than the desired flow rate. Therefore, the controller again adjusts the voltage signal, this time by applying a negative voltage and/or a positive voltage of a lesser magnitude, in an attempt to range in to the desired flow rate. This process, which continues in a decreasing oscillatory manner, is generally illustrated as 202 in FIG. 2 and continues for approximately 1.5 seconds until the MFC ranges in on approximately a 0.3 volt voltage signal being applied to the valve driver, which may correspond to the desired 20 sccm gas flow, which is shown as 203.
The rate at which MFC""s range in on the valve driver voltage that corresponds to the desired gas flow is generally a function of the electrical gain characteristics of the circuitry in the MFC. Therefore, the ranging rate and/or gain characteristics are generally not variables that the user may manipulate in order to achieve a quicker gas flow response time. As such, MFCs generally range in on the desired gas flow rate in a relatively constant time frame incorporated into the respective MFC via the controller circuit gain characteristics. Although the characteristics associated with this predetermined gain parameter may be acceptable for various semiconductor processing techniques, many chemical and thermal based semiconductor processing techniques are extremely sensitive to initial gas flow parameters. As such, an MFC configured with the above noted gain and/or ranging characteristics may generate an inconsistent and/or degraded processing environment upon activation of the initial gas flow through the MFC. These characteristics, even though they may only be present for a few seconds after initial gas flow is initiated, are generally sufficient to destroy device characteristics of devices manufactured by sensitive thermal and/or chemical reactions, as improper proportions of reactant gases may create undesirable chemical reaction. Another related disadvantage of conventional MFCs is that the ramp rate is not generally controllable, i.e., MFCs are generally preprogrammed for a specific ramp rate which eliminates the option of allowing the user to control/tune this parameter in a semiconductor processing system.
Therefore, there is a need for an improved MFC capable of initiating a gas flow for a semiconductor processing chamber. Further, there is a need for an improved MFC, wherein the MFC is capable of initiating gas flow by immediately opening a flow control valve to a predetermined position that is known to correspond to a specific gas flow. Further still, there is a need for an improved MFC, capable of storing information from previous flow control valve positions and flow rates in order to accurately determine an initial starting position of the flow control valve within the MFC in current flow control situations. Further still, there is a need for a tunable ramp rate circuit for a mass flow controller, wherein the tunable circuit allows a user to fine tune the rate at which an MFC ramps up to a predetermined gas set point. Further still, there is a need for a soft start timing circuit that may be implemented in conjunction with a soft start enabled MFC, wherein the timing circuit is configured to delay the opening of the MFC flow control valve after primary valves of a system have been opened and to bias the flow control valve to a hard closed position until the delay period has expired.
Embodiments of the invention generally provide a method and apparatus for stabilizing startup gas flow through an MFC supplying reactant gases to a semiconductor system, wherein the method includes receiving a gas setpoint in a tunable activation circuit, determining a desired valve control voltage corresponding to the gas setpoint, and bypassing an MFC valve controller signal. The method further includes ramping a valve control voltage for a flow control valve of the MFC to the desired valve control voltage with the tunable activation circuit, and then returning control of the MFC to an MFC valve controller circuit.
Embodiments of the invention further provide an apparatus for stabilizing gas flow into a semiconductor processing system during startup conditions, the apparatus including a mass flow controller having an electronic setpoint output, a gas flow input, a flow control valve, and a regulated gas flow output. The apparatus further includes a tunable activation circuit device in communication with the flow control valve, wherein the tunable activation circuit device is configured to override the mass flow controller set point output during a gas flow startup process and provide a selectively tunable valve ramp voltage thereto during the flow startup process.
Embodiments of the invention further provide a tunable activation circuit device for a mass flow controller including a first input for receiving a signal corresponding to a desired flow rate from a user, a second input for receiving a set point voltage from a mass flow controller control circuit, and a valve control voltage generator in communication with the first input. The tunable activation circuit device further includes a data storage device in communication with the valve control voltage generator, the data storage device having parameters corresponding to previous flow rates stored therein, a switching device in communication with the first and second inputs, the switching device being configured to switch between the second input and an output of the valve control voltage generator, and an output in communication with the switching device.